Underbalanced drilling (UBD) has gained strong momentum in recent years because of a number of advantages of the technology including reduced formation damage and minimized lost circulation. Due to the complex nature of water, oil, gas and solid multiphase flow in the UBD systems, numerous runs of sophisticated computer programs are required to draw the boundary of the safe gas -liquid rate envelope. It is highly desirable to have a simple and reliable procedure to perform optimum UBD designs.
This paper describes an innovative procedure to delineate the boundary of the safe gas -liquid rate envelope for UBD flow rates. In developing the safe gas-liquid rate envelope, formation fluid pressure limits the upper bound of the flowing bottom hole pressure and wellbore collapse pressure serves the lower bound of the circulation-break bottom hole pressure. The envelope is closed with boundaries determined by fluid’s cutting carrying capacity and wellbore washout criteria. Detailed procedure for the development of the safe gas-liquid rate envelope using a spreadsheet program is described in the paper. A successful field UBD case is reviewed and compared with the safe gas -liquid rate envelope. This work provides drilling engineers and drilling supervisors an easy-to-use approach to designing and mo difying gas and liquid injection rates in UBD. The safe gas-liquid rate envelope can also be used for evaluating feasibility of UBD under given geological conditions.